1. Field of Invention
This invention relates to aluminum alloys suitable for use in aircraft, automobiles, and other applications and to improved methods of producing such alloys. More specifically, it relates to a method of making an improved aluminum product, particularly useful in aircraft applications, having improved damage tolerant characteristics, including improved corrosion resistance, formability, fracture toughness and strength properties.
2. Description of the Related Art
Workers in the field have used heat treatable aluminum alloys in a number of applications involving relatively high strengths such as aircraft fuselages, vehicular members and other applications. Aluminum alloys 6061 and 6063 are among the most popular heat treatable aluminum alloys in the United States. These alloys have useful strength and toughness properties in both T4 and T6 tempers. They lack, however, sufficient strength for most structural aerospace applications.
More recently, Alloys 6009 and 6010 have been used as vehicular panels in cars and boats. These alloys and their products are described in U.S. Pat. No. 4,082,578, issued Apr. 4, 1978 to Evancho et al. In general, alloy 6010 includes 0.8 to 1.2 wt. % Si, 0.6 to 1.0% Mg, 0.15 to 0.6 wt. % Cu, 0.2 to 0.8 wt. % Mn, balance essentially aluminum. Alloy 6009 is similar to alloy 6010 except for lower Si at 0.6 to 1.0 wt. % and lower Mg at 0.4 to 0.6 wt. %.
In spite of the usefulness of the 6009 and 6010 alloys, these alloys are generally unsuitable for the design of commercial aircraft which require different sets of properties for different types of structures. Depending on the design criteria for a particular airplane component, improvements in fracture toughness and fatigue resistance result in weight savings, which translate to fuel economy over the lifetime of the aircraft, and/or a greater level of safety.
To meet this need, workers in the field have attempted to develop alloys having improved impact and dent resistance as well as substantial toughness. For example in U.S. Pat. No. 4,589,932, issued May 20, 1986 to Park describes a 6013 alloy which includes 0.4 to 1.2 wt. % Si, 0.5 to 1.3 wt. % Mg, 0.6 to 1.1 wt. % Cu, 0.1 to 1% Mn, the balance essentially aluminum. Similarly, Japanese Patent Application Kokai No. 60-82643 describes an alloy which includes 0.4 to 1.5 wt. % Si, 0.5 to 1.5 wt. % Mg, 0.4 to 1.8 wt. % Cu, 0.05 to 1.0 wt. % Mn, 1.0 to 6.0 wt. % Zn which emphasizes adding copper to reduce intercrystalline cracks. These new generation of 6XXX alloys are characterized by relatively high copper levels which provide a strength advantage. Unfortunately, the high copper contents also produce an increased susceptibility to intergranular corrosion. Corrosion of this type causes strength degradation in service, but more importantly, greatly detracts from fatigue resistance.
Corrosion damage has been a perennial problem in today's aircraft, and the fuselage is the prime location for corrosion to occur. Improvements in corrosion resistance, therefore, are often sought with or without weight savings. Thus, the new generation of 6XXX alloys are generally unsuitable for aircraft applications because of their susceptibility to intergranular corrosion caused by high copper levels as discussed in Chaudhuri et al., Comparison of Corrosion-Fatigue Properties of 6013 Bare, Alclad 2024, and 2024 Bare Aluminum Alloy Sheet Materials, JMEPEG (1992) 1:91-96.
Another approach taken in U.S. Pat. No. 4,231,817, issued Nov. 4, 1980 to Takeuchi et al. and Japanese Patent Application Kokai Nos. 55-8426 and 53-65209 which generally describe 6061 and 6063 type alloys which have added zinc. Although the added zinc is reported to improve corrosion resistance, these alloys lack sufficient strength for most structural aerospace applications.
Turning now to formability, many aerospace alloys such as 2024 and 7075 are formed in the annealed O temper or freshly quenched W temper. Forming in the O temper requires, however, a subsequent solution heat treatment operation, which usually introduces distortion problems. Forming in the W temper alleviates the distortion concern, but sheet in this condition hardens as it naturally ages, so either the delay time between solution heat treating and forming must be minimized, or the material must be stored in a freezer until it is ready to be formed. In contrast, a sheet material that has good formability in the stable T4 condition circumvents all of these potential problems because the manufacturer need only age to the T6 temper after making the part. It is therefore desirable for the aerospace alloy to have good formability in the stable T4 condition.
In sum, a need remains for an alloy having improved resistance to corrosion and yet maintains the desirable strength, toughness, and T4 formability properties exhibited by the 6013 type alloys. Accordingly, it is an object of this invention to provide such an alloy.